Generally, in a vehicle interior, speakers are provided at a plurality of positions. For example, a left front speaker and a right front speaker are provided at positions symmetrical to each other with respect to a central line of an in-vehicle space. However, if a listening position of a listener (a driver seat, a front passenger seat, a rear seat or the like) is considered as a reference position, these speakers are not positioned symmetrically. Therefore, due to differences in distances between the listening position of the listener and each of a plurality of speakers (difference in traveling time between playback sounds output from respective speakers), sound image localization bias by Haas effect occurs.
For example, Japanese Patent Provisional Publication No. H7-162985A (hereinafter, “patent document 1”) discloses an apparatus that is capable of remedying the sound image localization bias. The apparatus disclosed in patent document 1 suppresses the sound image localization bias by adjusting time such that playback sounds emitted from all of the speakers reach the listener at the same time (i.e., a time alignment process). More specifically, the apparatus disclosed in patent document 1 corrects, over the entire range, the sound image localization bias and frequency characteristic disorders due to phase interferences by dividing an audio signal into a high range and a low range using a band dividing circuit and then adjusting time of each of playback sounds to be emitted from each of low band speakers and high band speakers.
However, the apparatus disclosed in patent document 1 has a problem that linearity of transmission characteristic at the listening position of the listener degrades due to loss of signals and double additions that occur in the band dividing circuit. Furthermore, the apparatus disclosed in patent document 1 also has a problem that peaks and/or dips occur in frequency characteristic around a crossover frequency when mixing the signals divided by the band dividing circuit.
In view of above, a brochure of International Patent Publication No. WO2009/095965A1 (hereinafter, “patent document 2”) proposes an apparatus for performing a time alignment process that is capable of improving linearity of transmission characteristic at the listening position of the listener and suppressing occurrence of peaks and/or dips in frequency characteristic when mixing.
The apparatus disclosed in patent document 2 uses a digital filter to improve linearity of transmission characteristic at the listening position of the listener. More specifically, the apparatus disclosed in patent document 2 uses an FIR (Finite Impulse Response) filter. The FIR filter disclosed in patent document 2 is a high order filter having a steep cutoff frequency to suppress the occurrence of dips, and has a configuration which needs a plurality of delay circuits and multipliers. In particular, when constituting an FIR filter having a linear phase characteristic in which the phase is constant in all of the frequency bands, a plurality of delay circuits and multipliers are needed, and therefore a problem arises that a processing load increases. Further, the configuration described in the patent document 2 has a problem that as the number of divided frequency bands increases (as the frequency band for which time is adjusted becomes smaller), the required delay circuits and the multipliers further increase and thereby the processing load further increases.
Japanese Patent Provisional Publication No. 2015-12366A (hereafter, referred to as “patent document 3”) describes an apparatus that executes a time alignment process for adjusting propagation delay times of a plurality of frequency bands. This apparatus is suitably configured to reduce the processing load while also reducing occurrence of dips. Specifically, the apparatus described in the patent document 3 generates a phase control signal relating to phase control (the phase rotation and the phase offset) for each of the frequency bands of an audio signal and smoothly connects the phase change between the frequency bands by executing a smoothing process for the generated phase control signal, so that occurrence of dips is reduced in a configuration not requiring many FIR filters (i.e., a configuration where the processing load is reduced).